The present invention relates to electroluminescence elements.
Organic electroluminescence elements (hereinafter, referred to as “organic EL elements”) are employed in displays and lighting devices as light emission elements. One such element basically includes an anode electrode layer, an organic light emission layer, and a cathode electrode layer, which are deposited on a transparent substrate formed of, for example, glass in this order.
The organic EL element emits light when voltage is applied between the anode electrode layer and the cathode electrode layer and thus an electric current flows in the organic light emission layer. Typically, the light emitted by the organic light emission layer passes through an electrode of the anode or cathode electrode layer and is thus sent to the exterior. In this case, at least the electrode through which the light passes is formed by a transparent electrode that transmits the light. The transparent electrode is formed of, typically, ITO (indium tin oxide) or ZnO (zinc oxide), which has a higher volume resistivity than that of an electrode formed of metal, such as aluminum or silver.
The brightness of the organic EL element is influenced by the current density of the organic light emission layer and becomes greater as the current density becomes higher. If the transparent electrode is formed of ITO, the difference between the electric resistance value from a power supply terminal to a portion of the electrode closer to the power supply terminal and the electric resistance value from the power supply terminal to a portion of the electrode far from the power supply terminal becomes relatively large. This results in a relatively great difference between the current densities of the corresponding portions of the organic light emission layer. Accordingly, the brightness of the light emission zone of the organic EL element may become non-uniform.
In one method of adjusting the brightness distribution of the organic EL element, as described in Japanese Laid-Open Patent Publication No. 2003-123990, a transparent electrode is combined with an auxiliary electrode that is formed of metal with relatively high electric conductivity. More specifically, as shown in FIG. 15, the method employs an auxiliary electrode 52 including a contact portion. The contact portion has three sides that are held in contact with a transparent anode electrode layer 51, which is deposited on a substrate 50.
In this method, the auxiliary electrode 52 is provided around the transparent anode electrode layer 51. The brightness distribution of the organic EL element is thus adjusted by lowering the electric resistance of a portion of the transparent anode electrode layer 51 corresponding to the auxiliary electrode 52. However, even in the auxiliary electrode 52, the difference between the electric resistance value from a power supply terminal to a portion of the auxiliary electrode 52 closer to the power supply terminal and the electric resistance value from the power supply terminal to a portion of the auxiliary electrode 52 far from the power supply terminal becomes relatively large. This decreases the current density of a portion of the organic light emission layer far from the power supply terminal, lowering the brightness of the portion correspondingly.
Similar problems occur also in inorganic EL elements.